1. Field of the Invention
The present invention relates particularly to an amplification circuit which provides a high gain even with a low-voltage power supply.
2. Description of the Background Art
FIG. 13 is a circuit diagram showing a conventional grounded-source amplification circuit, in which a negative-feedback impedance element is connected to the source. In FIG. 13, M1 denotes an N-channel MOS transistor, Z1, ZD, and ZS each denote an impedance element having a DC path, input terminal to which a circuit in the preceding stage (not shown) is connected, RFin denotes a high-frequency signal to be amplified that propagated from the input terminal P1, P2 denotes an output terminal to which a circuit in the following stage (not shown) is connected, RFout denotes an output signal propagating to the output terminal P2, Vgs denotes the voltage at the gate with respect to the source (the gate-source voltage), and Id denotes the drain current.
An impedance element represents a resistance, a capacitor, an inductor, or a combination thereof. A DC path represents an impedance element whose both ends are connected in a DC manner, such as a resistance.
First, the structure of the conventional amplification circuit will be described. The MOS transistor M1 has its gate electrically connected to the input terminal P1, its source electrically connected to ground through the impedance element ZS, its drain connected to a supply voltage VDD through the impedance element ZD and then the impedance element Z1, and its back gate connected to ground. The output terminal P2 is electrically connected to a connection point between the impedance element Z1 and the impedance element ZD.
The source, drain, and gate of the MOS transistor M1 are biased to potentials set so that the MOS transistor M1 operates in the ON region, i.e., in the saturation region or linear region.
Next, the operation of the conventional amplification circuit will be described. Generally, the electric characteristic of the MOS transistor M1 is given by the following equation (1): ##EQU1##
The character .beta. is a constant which is determined by manufacture process and device structure (a transconductance constant), and VT is the threshold voltage. In the conventional device, the threshold voltage VT is always constant (=VT0), since the back gate is biased to ground.
The gain AV can be given by the following equation (2) by using the equation (1): ##EQU2##
According to the equation (2), a high gain AV can be obtained by increasing the impedance Z1 or the gate-source voltage Vgs.
However, if the impedance Z1 is increased too large, the potential at the drain will become so low that the MOS transistor M1 cannot perform amplifying operation. Further, it is difficult to bias the gate-source voltage Vgs over the supply voltage VDD in a general semiconductor integrated circuit. Accordingly, when the MOS transistor M1 is intended for amplifying operation, the impedance element Z1 and the gate-source voltage Vgs must be set within a limit defined by the supply voltage VDD. Accordingly, as the supply voltage VDD is lower, the impedance element Z1 and the gate-source voltage Vgs cannot be made larger, so that a high gain AV cannot be obtained.
For example, under the condition of supply voltage VDD=0.5 V and threshold voltage VT=0.35 V, the MOS transistor M1 can perform the amplifying operation but the gain AV in this case will be low.